The present invention relates, in general, to a method and device for producing a pulsed voltage which is pulse-duration modulated in a control system for setting the position of a mechanical element. In particular the invention relates to a method of producing a pulsed setting voltage, which is pulse-duration modulated with an electric control value, for application of a setting drive having an inductance, particularly an electric motor setting drive of a throttle valve of an internal combustion engine. Also in particular the invention relates to a device for producing a pulsed pulse-duration modulated setting voltage for the operation of a setting drive which has an inductance, particularly an electric motor setting drive of a throttle valve of an internal combustion engine.
Electrically activated setting drives for regulating and control systems are frequently acted on by a pulsed setting voltage which is pulse-duration modulated in accordance with the setting output. The use of the pulse-duration-modulated setting voltage has an advantage, for instance, over the use of a continuous constant voltage of adjustable value in that the system cost is low, and also in that power loss in a pulse generator of the system can be kept relatively low. In the use semiconductor elements which in a power stage of a generator of a pulsed setting voltage, for all practical purposes, short-duration power losses occur only during leading and trailing edges of a pulse. Furthermore, in accordance with the prior art, the pulsing of the setting voltage is advantageously used in order periodically to mechanically excite the setting device so as to reduce or eliminate the influences of mechanical hysteresis which impairs precision.
For the last-mentioned purpose, particularly when using an electric motor setting drive for the actuating of a throttle valve of an internal combustion engine, a pulse frequency of 130 to 160 Hz is customary. The clock frequency should not be too high since, in such case, it may no longer exert a sufficient exciting action due to the mechanical damping of a setting element.
On the other hand, the electric power loss in a setting drive whose equivalent circuit can be regarded as an inductance and a resistance is greater with lower clock frequencies of the pulsed setting voltage. This is due to the fact that the power loss increases with the square of the setting current, and that the variations or ripple of the setting current, which is smoothed by the inductance of the setting drive, is greater with lower clock frequencies of the setting voltage. In other words, if one wants to obtain the highest possible useful output power with a setting drive of a given type and size, a high clock frequency is advisable in order not to exceed a limit value for the power loss.
For the reasons described above, the clock frequency with which a pulsed setting voltage is produced is based on a compromise between conflicting requirements.
In order that a setting drive can fulfill its function, in a regulated or controlled system, of following a control or regulating variable, the setting voltage is pulse-duration modulated by an electric control variable.
In pulse-duration modulation of a setting voltage, the pulse duty factor is influenced to obtain a larger or smaller arithmetic mean value of the setting voltage for control of torque produced by the setting drive.